AC-DC Electric power converting station

ABSTRACT

An AC-DC electric power converting station is provided wherein circuit lines consisting of converter transformers/ positive and negative polarity valve structures/ DC reactors/ DC circuit breakers are of L-shape and inverted L-shape. The positive and negative polarity valve structures are so arranged that their surfaces in the longitudinal direction face each other. The converting transformers are arranged along the other surfaces of the positive and negative polarity valve structures in the longitudinal direction thereof. The DC reactors and DC circuit breakers are series-connected in the vicinities of the outputs in the direction of width of the positive and negative polarity valve structures. AC and DC switchgears of gas insulated type are arranged to face each other so that they sandwich the positive and negative polarity valve structures therebetween. Conduit air cables connect the positive and negative polarity valve structures and the AC and DC switchgears.

BACKGROUND OF THE INVENTION

The present invention relates to an AC-DC electric power converting station in a DC transmission system. More particularly, the present invention relates to an AC-DC electric power converting station wherein constituting equipment is so arranged that the land area for the station may be greatly reduced.

Electric power systems have recently been required to transmit increasingly great amounts of electric power. The tendency is to use a high transmission voltage for DC transmission to a remote place, as in the case of AC transmission. For example, some AC-DC electric power converting stations in current operation are of ±250 KV class. In the prior art AC-DC electric power converting stations, the arrangement of equipment is generally determined based on the utilization of air insulation.

For transmission of power in the 10,000 MW class, constituting equipment of UHV (ultra high voltage) class is required for both AC transmission systems and DC transmission systems. In the case of UHV class transmission, the necessary air insulation distance separating adjacent transmission lines supported by transmission pylons becomes great. Accordingly, the land area occupied by the transmission lines, and the land area for the transmission facility, also become great. For these reasons, DC transmission, which requires a smaller air insulation distance, is advantageous over AC transmission in that the required land area may be reduced and effective insulation may be easily established.

For example, in the case of 10,000 MW DC transmission, requirements for transmission capacity may be satisfied by equipment of ±500 KV class, so only insulation corresponding to 500 KV AC transmission need be performed.

When, as in the case of the prior art AC-DC electric power converting stations, the arrangement of the constituting equipment of the station of ±500 KV class is determined based on the utilization of air insulation, the land area necessary for such a station may be slightly reduced from that necessary for the AC transmission but nevertheless still remains great.

As a solution to this problem, it is possible to gas-insulate the switchgears of the station to make them compact in size, to thereby reduce the land area required for the station.

However, reducing the size of the switchgears does not significantly contribute to reduction of the overall station size.

An arrangement of equipment based on the utilization of air insulation as used with a prior art AC-DC electric power converting station of 10,000 MW class will now be described with reference to FIGS. 1 and 2.

FIG. 1 is a single line diagram of a prior art AC-DC electric power converting station. Circuit breakers 12₁ to 12₄ are arranged in a double bus/four bus tie system to AC side buses 10₁ and 10₂. The first and second circuit breakers 12₁ and 12₂ are arranged for the AC side buses 10₁ and 10₂, respectively, and the buses 10₁ and 10₂ are connected to each other through both the third and fourth circuit breakers 12₃ and 12₄. Disconnecting switches 14₀ and 16₀ are arranged at opposite sides of the third circuit breaker 12₃. Disconnecting switches 14₁₉ and 16₁₉ are similarly arranged at opposite sides of the fourth circuit breaker 12₄.

The buses 10₁ and 10₂ are connected to an AC transmission line (not shown) via a first cable head 20₁ through a disconnecting switch 22₁, a circuit breaker 24₁, and disconnecting switches 14₂ and 16₂. Similarly, the buses 10₁ and 10₂ are connected to the AC transmission line described above via second to fourth cable heads 20₂ to 20₄, respectively, as follows: through a disconnecting switch 22₂, a circuit breaker 24₂, and disconnecting switches 14₆ and 16₆ ; through a disconnecting switch 22₃, a circuit breaker 24₃, and disconnecting switches 14₁₃ and 16₁₃ ; and through a disconnecting switch 22₄, a circuit breaker 24₄, and disconnecting switches 14₁₇ and 16₁₇.

A first phase modifier 26₁ is connected to the buses 10₁ and 10₂ through a circuit breaker 28₁ and disconnecting switches 14₁ and 16₁, and a second phase modifier 26₂ is connected to the buses 10₁ and 10₂ through a circuit breaker 28₂ and disconnecting switches 14₁₈ and 16₁₈.

A first converter 30₁ comprises thyristor valves 32₁ to 32₄. The valves 32₁ and 32₂ constitute a high voltage stage 34₁, and the valves 32₃ and 32₄ constitute a low voltage stage 34₂. The high voltage stage 34₁ is connected to the output of a first converter transformer 36₁, and the low voltage stage 34₂ is connected to the output of a second converter transformer 36₂. The inputs of the first and second converter transformers 36₁ and 36₂ are commonly connected and are connected to the buses 10₁ and 10₂ through a circuit breaker 38₁ and disconnecting switches 14₃ and 16₃.

A second converter 30₂ comprises thyristor valves 32₅ to 32₈ and has a negative low voltage stage 34₃ and a negative high voltage stage 34₄. The negative low voltage stage 34₃ and the negative high voltage stage 34₄ are respectively connected to the outputs of third and fourth converter transformers 36₃ and 36₄. The inputs of the third and fourth converter transformers 36₃ and 36₄ are commonly connected and are connected to the buses 10₁ and 10₂ through a circuit breaker 38₂ and connecting switches 14₅ and 16₅.

An AC filter 40₁ is connected to the buses 10₁ and 10₂ through a circuit breaker 38₃ and disconnecting switches 14₄ and 16₄.

The cathode of the thyristor valve 32₁ is connected to DC side positive bus lines 50₁ and 50₂ through a first DC reactor 42₁, a circuit breaker 44₁, and disconnecting switches 46₁ and 48₁. In addition, the anode of the thyristor valve 32₈ is connected to DC side negative bus lines 56₁ and 56₂ through a second DC reactor 42₂, a circuit breaker 44₂, and disconnecting switches 52₁ and 54₁.

The anode of the thyristor valve 32₄ is connected to the cathode of the thyristor valve 32₅ through disconnecting switches 58₁ and 60₁. A node of the disconnecting switches 58₁ and 60₁ is connected to a neutral conductor N through a disconnecting switch 62₁.

The first to fourth converter transformers 36₁ to 36₄, the first and second converters 30₁ and 30₂, the first and second DC reactors 42₁ and 42₂, and the circuit breakers 44₁ and 44₂ constitute a first unit converting apparatus 64₁.

The constructions of second to fourth unit converting apparatuses 64₂ to 64₄ are the same as that of the first converting apparatus 64₁. The second converting apparatus 64₂ is connected to the buses 10₁ and 10₂ through a circuit breaker 38₄ and disconnecting switches 14₇ and 16₇ ; a circuit breaker 38₆ and disconnecting switches 14₈ and 16₈ ; and a circuit breaker 38₅ and disconnecting switches 14₉ and 16₉. The third converting apparatus 64₃ is connected to the buses 10₁ and 10₂ through a circuit breaker 38₇ and disconnecting switches 14₁₀ and 16₁₀ ; a circuit breaker 38₉ and disconnecting switches 14₁₁ and 16₁₁ ; and a circuit breaker 38₈ and disconnecting switches 14₁₂ and 16₁₂. The fourth converting apparatus 64₄ is connected to the buses 10₁ and 10₂ through a circuit breaker 38₁₀ and disconnecting switches 14₁₄ and 16₁₄ ; a circuit breaker 38₁₂ and disconnecting switches 14₁₅ and 16₁₅ ; and a circuit breaker 38₁₁ and disconnecting switches 14₁₆ and 16₁₆.

To DC side positive bus lines 50₁ and 50₂ are connected a high voltage stage (not shown) of the second unit converting apparatus 64₂ through a circuit breaker 44₃ and disconnecting switches 46₃ and 48₃ ; a high voltage stage (not shown) of the third unit converting apparatus 64₃ through a circuit breaker 44₅ and disconnecting switches 46₅ and 48₅ ; and a high voltage stage (not shown) of the fourth unit converting apparatus 64₄ through a circuit breaker 44₇ and disconnecting switches 46₆ and 48₆.

To DC side negative bus lines 56₁ and 56₂ are connected a negative high voltage stage (not shown) of the second unit converting apparatus 64₂ through a circuit breaker 44₄ and disconnecting switches 52₂ and 54₂ ; a negative high voltage stage (not shown) of the third unit converting apparatus 64₃ through a circuit breaker 44₆ and disconnecting switches 52₄ and 54₄ ; and a negative high voltage stage (not shown) of the fourth unit converting apparatus 64₄ through a circuit breaker 44₈ and disconnecting switches 52₆ and 54₆.

The DC side positive bus lines 50₁ and 50₂ are connected to each other at one end through a circuit breaker 58 and disconnecting switches 46₇ and 48₇ and are of the double bus system. The DC side negative bus lines 56₁ and 56₂ are connected to each other at one end through a circuit breaker 60 and disconnecting switches 52₀ and 54₀ and are of the double bus system.

The DC side positive bus lines 50₁ and 50₂ are connected to a positive DC transmission line (not shown) via a DC side first cable head 64₁ through disconnecting switches 46₂ and 48₂ and a circuit breaker 62₁ and via a DC side second cable head 64₂ through disconnecting switches 46₄ and 48₄ and a circuit breaker 62₂. The neutral conductor N is connected to a neutral transmission line (not shown) via DC side third and fourth cable heads 64₃ and 64₄ through circuit breakers 62₃ and 62₄, respectively. The DC negative bus lines 56₁ and 56₂ are connected to a negative DC transmission line (not shown) via a DC side fifth cable head 64₅ through disconnecting switches 52₃ and 54₃ and a circuit breaker 62₅ and via a DC side sixth cable head 64₆ through disconnecting switches 52₅ and 54₅ and a circuit breaker 62₆.

A first DC filter 66₁ is inserted between the DC side first and third cable heads 64₁ and 64₃. A second DC filter 66₂ is inserted between the sixth and third cable heads 64₆ and 64₃. A third DC filter 66₃ is inserted between the DC side second and fourth cable heads 64₂ and 64₄. A fourth DC filter 66₄ is connected between the DC side fifth and fourth heads 64₅ and 64₄.

The disconnecting switches 22₁ to 22₄, the circuit breakers 24₁ to 24₄, the AC side buses 10₁ and 10₂, the disconnecting switches 14₀ to 14₁₉ and 16₀ to 16₁₉, the circuit breakers 38₁ to 38₁₂, the circuit breakers 28₁ and 28₂, and the circuit breakers 12₁ to 12₄ constitute an AC side gas insulated switchgear 68. The DC side positive bus lines 50₁ and 50₂, the disconnecting switches 46₁ to 46₇ and 48₁ to 48₇, the DC side negative bus lines 56₁ and 56₂, the disconnecting switches 52₀ to 52₆ and 54₀ to 54₆, the circuit breakers 60 and 62₁ to 62₆, and the first to fourth DC filters 66₁ to 66₄ constitute a DC side gas insulated switchgear 70.

FIG. 2 schematically shows an example of the arrangement of the constituent equipment of the prior art AC-DC electric power converting station shown by the single line diagram of FIG. 1 according to the air insulation system. The same reference numerals as in FIG. 1 denote the same parts. Referring to FIG. 2, a radiator 72 dissipates heat generated by the converter transformers 36₁ to 36₄ and so on for cooling.

When the arrangement of equipment is accomplished based on the utilization of air insulation, a land area of about 770 m×510 m=392,700 m² is required, as shown in FIG. 2.

As may further be seen from FIG. 2, a wider space is required for replacement of equipment in case of breakdown of the thyristor valves 32₁ to 32₈ and so on.

SUMMARY OF THE INVENTION

It is, therefore, the primary object of the present invention to provide an AC-DC electric power converting staton which retains the functionality of the prior art AC-DC electric power converting station while it secures the space necessary for part replacement in case of an accident such as breakdown of parts, and which enables a great reduction in the land area required for the station.

This object has been attained by the AC-DC electric power converting station which includes a gas-insulated AC side switchgear having first and second longitudinal sides opposite each other, and first and second transverse sides opposite each other; a gas-insulated DC side switchgear having third and fourth transverse sides opposite each other, and arranged opposite to the gas-insulated AC side switchgear with the third longitudinal side facing the second longitudinal side of the AC side switchgear; at least one first group of semiconductor switching elements, each group enclosed within a first valve structure of positive polarity, which has fifth and sixth longitudinal sides opposite each other, and fifth and sixth transverse sides opposite each other, and which is arranged between the AC and DC side switchgears with the fifth transverse side parallel to the second longitudinal side of the AC side switchgear, and the sixth transverse side parallel to the third longitudinal side of the DC side switchgear; at least one second group of semiconductor switching elements connected to the first group of semiconductor switching elements, each group enclosed within a second valve structure of negative polarity which has seventh and eighth longitudinal sides opposite each other, and seventh and eighth transverse sides opposite each other, and which is so arranged that the seventh transverse side is parallel to the second longitudinal side of the AC side switchgear, the eighth transverse side is parallel to the third longitudinal side of the DC side switchgear, and the seventh longitudinal side faces the sixth longitudinal side of the first valve structure; first and second converter transformers respectively connected to the first and second groups of semiconductor switching elements, and respectively arranged in opposition to the fifth and eighth longitudinal sides of the first and second valve structures; first and second DC circuit breakers respectively connected to the first and second groups of semiconductor switching elements, and respectively arranged in opposition to the sixth and eighth transverse sides of the first and second valve structures, and between the DC side switchgear and the pair of first and second valve structures; first and second DC reactors respectively connected between the first group of semiconductor switching elements and the first DC circuit breaker and between the second group of semiconductor switching elements and the second circuit breaker, the first DC reactor arranged between the first valve structure and the first DC circuit breaker, and the second DC reactor arranged between the second valve structure and the second circuit breaker; first conduit cables extending to connect the first and second valve structures to the AC side switchgear; second conduit cables extending to connect the first and second valve structures to the DC side switchgear.

With the equipment arrangement of the AC-DC electric power converting station of the present invention, the circuit line formed by the converter transformers, for the valve structures of positive polarity/the valve structures of positive polarity/the DC reactors/and the DC circuit breakers, is of L-shape; and the circuit line formed by the converter transformers, for the valve structures of negative polarity/the valve structures of positive polarity/the DC reactors/and the DC circuit breakers, is of inverted L-shape. Due to this arrangement, the space required for responding to an accident (e.g., replacement of damaged equipment or the like) may be greatly reduced as compared with that required by the prior art station. Furthermore, since the conduit cables are used for the lines between the AC and DC side switchgears and the valve structures of positive and negative polarities, the space which has been hitherto required for these cables may also be eliminated. In accordance with the present invention, the land area for the AC-DC electric power converting station may be greatly reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example and to make the description clearer, reference is made to the accompanying drawings in which:

FIG. 1 is a single line diagram of a prior art AC-DC electric power converting station;

FIG. 2 is a view showing the arrangement of the equipment of the prior art AC-DC electric power converting station of the circuit of FIG. 1 according to the air insulation system; and

FIG. 3 is a view showing the arrangement of the equipment of an AC-DC electric power converting station according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a view showing the arrangement of the equipment of an AC-DC electric power converting station according to an embodiment of the present invention which is accomplished based on the single line diagram of the prior art AC-DC electric power converting station shown in FIG. 1. The same reference numerals denote the same parts as in FIG. 1.

A positive polarity valve structure 74₁ houses the positive side thyristor valves 32₁ to 32₄ of air insulation type. A negative polarity valve structure 74₂ houses the negative side thyristor valves 32₅ to 32₈ of air insulation type. The thyristor valves of the low voltage stage 34₂ and the high voltage stage 34₁ are series-connected in the longitudinal direction of the valve structure 74₁. Similarly, the thyristor valves of the low voltage stage 34₃ and the high voltage stage 34₄ are series-connected in the longitudinal direction of the valve structure 74₂. Therefore, as shown in the figure, the low voltage stages 34₂ and 34₃ and the high voltage stages 34₁ and 34₄ of the valve structures 74₁ and 74₂ are arranged to oppose each other. The positive side thyristor valves 32₁ to 32₄ are only required to be disposed separately from the negative side thyristor valves 32₅ to 32₈. Therefore, the positive polarity valve structure 74₁ housing the positive side thyristor valves 32₁ to 32₄ and the negative polarity valve structure 74₂ housing the negative side thyristor valves 32₅ to 32₈ may be connected with a connecting housing portion or share a wall which separates them from each other to form a larger housing. Also the separating wall is not required. DC output terminals of the two groups of thyristors are so arranged to be located at the right ends of the valve structures 74₁ and 74₂ in FIG. 3.

The converter transformers 36₂ and 36₁, respectively, of the low voltage stage 34₂ and the high voltage stage 34₁ of the valve structure 74₁ are arranged along the outer surface in the longitudinal direction of the valve structure 74₁. Similarly, the converter transformers 36₃ and 36₄, respectively, of the low voltage stage 34₃ and the high voltage stage 34₄ of the valve structure 74₂ are arranged along the outer surface in the longitudinal direction of the valve structure 74₂.

The DC reactor 42₁ connected to the high voltage stage 34₁ of the valve structure 74₁ is connected to the right side of the valve structure 74₁ in FIG. 3. The circuit breaker 44₁ is arranged to the right of the DC reactor 42₁. In the figure, a cooling tower 76₁ for the thyristor valves 32₁ to 32₄ is arranged in the vicinity above the circuit breaker 44₁.

The DC reactor 42₂ connected to the high voltage stage 34₄ of the valve structure 74₂ is arranged to the right of the valve structure 74₂ in the figure. The circuit breaker 44₂ is arranged to the right of the DC reactor 42₂. Referring to the figure, a cooling tower 76₂ for the thyristor valves 32₅ to 32₈ is arranged in the vicinity below the circuit breaker 44₂.

The paired valve structures 74₁ and 74₂, the converter transformers 36₁ to 36₄, the DC reactors 42₁ and 42₂, the DC circuit breakers 44₁ and 44₂, and the cooling towers 76₁ and 76₂ constitute a first unit converting apparatus 78₁.

Second to fourth unit converting apparatuses 78₂ to 78₄ are arranged in planar form in the vertical direction of the figure. The second to fourth unit converting apparatuses 78₂ to 78₄ are of the same construction as that of the first unit converting apparatus 78₁. The number of these unit converting apparatuses may be varied as necessary.

The DC side gas-insulated switchgear 70 is arranged to the right of and near these converting apparatuses 78₁ to 78₄. The AC side gas-insulated switchgear 68 is arranged to the left of and near these unit converting apparatuses 78₁ to 78₄.

AC filters 40₁ and 40₂, respectively, for the first and second unit converting apparatuses 78₁ and 78₂ are arranged in the vicinity above the AC side gas insulated switchgear 68 of the figure. The first phase modifier 26₁ is arranged to the left and near the AC filters 40₁ and 40₂. AC filters 40₃ and 40₄, respectively, for the third and fourth unit converting apparatuses 78₃ and 78₄ are arranged in the vicinity below the AC side gas insulated switchgear 68. The second phase modifier 26₂ is arranged to the left of and near the AC filters 40₃ and 40₄.

The DC outputs of the first to fourth unit converting apparatuses 78₁ to 78₄ are connected to the DC side gas-insulated switchgear 70 through DC side conduit cables 80₁ to 80₈ respectively. The neutral outputs of the first to fourth converting apparatuses 78₁ to 78₄ are connected to the DC side gas-insulated switchgear 70 through conduit cables 82₁ to 82₄ of the neutral conductor.

The inputs of the converter transformers 36₁ . . . of the first to fourth unit converting apparatuses 78₁ to 78₄ are connected to the AC side gas insulated switchgear 68 through AC side conduit cables 84₁ to 84₈.

In summary, according to this embodiment, the AC and DC switchgears 68 and 70 are gas insulated to reduce their bulks, and the conduit cables 80₁ to 80₈, 82₁ to 82₄, and 84₁ to 84₈ are used for the group of buses so that the space for accommodating them may be reduced to the minimum.

Further, according to this embodiment, the positive polarity valve structure 74₁ and negative polarity valve structure 74₂ are so arranged that their surfaces in the longitudinal direction face each other. The converter transformers 36₁ to 36₄ are arranged in the vicinities of the other surfaces of the valve structures 74₁ and 74₂ in the longitudinal directions thereof. The DC reactors 42₁ and 42₂ are series-connected to the right and near the valve structures 74₁ and 74₂ in FIG. 3. The DC circuit breakers 44₁ and 44₂ are respectively arranged to the right and near the DC reactors 42₁ and 42₂. Thus, the circuit line consisting of the converter transformers 36₁ and 36₂ /the valve structure 74₁ /the DC reactor 42₁ /and the DC circuit breaker 44₁ may be of L-shape. On the other hand, the circuit line consisting of the converter transformers 36₃ and 36₄ /the valve structure 74₂ /the DC reactor 42₂ /and the DC circuit breaker 44₂ may be of inverted L-shape. This arrangement facilitates replacement of damaged equipment in the case of an accident and allows reduction in the space as compared with the arrangement of the prior art station shown in FIG. 2. Thus, the land area required for the AC-DC electric power converting station of the embodiment incorporating the first to fourth unit converting apparatuses 78₁ to 78₄ of the constructions as described above is vastly reduced compared to that required for the prior art AC-DC electric power converting station shown in FIG. 2.

Describing the particular example of the land area required for the AC-DC electric power converting station of this embodiment, it is about 480 m in length and about 215 m in width as shown in FIG. 3. It follows that the land area of this embodiment is about 103,200 m². Accordingly, this land area is about 1/4 the land area (392,700 m²) of the prior art AC-DC electric power converting station shown in FIG. 2.

It is to be noted that the present invention is not limited to the particular embodiment described above, and various other modifications may be made within the spirit and scope of the present invention. 

What we claim is:
 1. An AC-DC electric power converting station comprising:a gas-insulated AC side switchgear having first and second longitudinal sides opposite each other, and first and second transverse sides opposite each other; a gas-insulated DC side switchgear having third and fourth longitudinal sides opposite each other, and third and fourth transverse sides opposite each other, and arranged opposite to said gas-insulated AC side switchgear with the third longitudinal side facing the second longitudinal side of said AC side switchgear; at least one first group of semiconductor switching elements, each group enclosed within a first valve structure of positive polarity, said first valve structure having fifth and sixth longitudinal sides opposite each other, and fifth and sixth transverse sides opposite each other, and arranged between said AC and DC side switchgears with the fifth transverse side parallel to the second longitudinal side of said AC side switchgear, and the sixth transverse side parallel to the third longitudinal side of said DC side switchgear; at least one second group of semiconductor switching elements connected to said first group of semiconductor switching elements, each group enclosed within a second valve structure of negative polarity, said second valve structure having seventh and eighth longitudinal sides opposite each other, and seventh and eighth transverse sides opposite each other, and so arranged that the seventh transverse side is parallel to the second longitudinal side of said AC side switchgear, the eighth transverse side is parallel to the third longitudinal side of said DC side switchgear, and said seventh longitudinal side faces the sixth longitudinal side of said first valve structure; first and second converter transformers respectively connected to said first and second groups of semiconductor switching elements, and respectively arranged in opposition to the fifth and eighth longitudinal sides of said first and second valve structures; first and second DC circuit breakers respectively connected to said first and second groups of semiconductor switching elements, and respectively arranged in opposition to the sixth and eighth transverse sides of said first and second valve structures, and between said DC side switchgear and said pair of first and second valve structures; first and second DC reactors respectively connected between said first group of semiconductor switching elements and said first DC circuit breaker and between said second group of semiconductor switching elements and said second circuit breaker, said first DC reactor arranged between said first valve structure and first DC circuit breaker, and said second DC reactor arranged between said second valve structure and said second circuit breaker; first conduit cables extending to connect said first and second valve structures to said AC side switchgear; second conduit cables extending to connect said first and second valve structures to said DC side switchgear.
 2. An AC-DC electric power converting station according to claim 1, wherein said first and second groups of semiconductor switching elements respectively comprise:series connections of a high voltage group and a lower voltage group.
 3. An AC-DC electric power converting station according to claim 1, further comprising:at least one AC filter connected to said AC side switchgear and arranged in opposition to at least one of the first and second transverse sides of said AC side switchgear.
 4. An AC-DC electric power converting station according to claim 1, further comprising:at least two cooling devices so arranged to be substantially parallel to the sixth and eighth transverse sides of said first and second valve structures, with said first and second DC circuit breakers interposed therebetween.
 5. An AC-DC electric power converting station according to claim 3, further comprising:phase modifiers so arranged to be in the direction of expansion of the first longitudinal side of said AC side switchgear, said at least one AC filter interposed between said phase modifiers and said pair of first and second valve structures. 